convert ajax to js

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C#

ASP.NET

jQuery

$.ajax
({
  type: "POST",
  url: "Default.aspx/CheckDateTime",
  data: "{'args':'" + tx_val + "'}",

  contentType: "application/json; charset=utf-8",
  dataType: "json",
  success: function(msg) 
  {
     msgbox.html(msg.d);
  }
});

I used this code in default.aspx and it is working fine.
Now I want to use it in js. How can I convert this code?